Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording medium (for example, a recording sheet, an OHP sheet, a cloth, or the like) using an electrophotographic system and relates to a process cartridge. Here, a process cartridge is a cartridge in which an electrophotographic photosensitive member (hereinafter referred to as photosensitive member) serving as an image bearing member is integrated with at least one of a charging means, developing means, and cleaning means as image forming process means acting on the photosensitive member. The process cartridge allows the cartridge to be detachably attached to an image forming apparatus body. Examples of an image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (for example, a laser beam printer, an LED printer, or the like), a facsimile apparatus, and a composite machine thereof.
Description of the Related Art
Conventionally, in an image forming apparatus which uses an electrophotographic system, first, charging means charges the surface of a photosensitive member to a desired potential and developing means causes developer to adhere to a latent image formed on the photosensitive member by the exposure means to form a developer image. Moreover, transfer means transfers the developer image to a recording medium, a fixing member fixes the image on the recording medium by applying heat and pressure to the recording medium, and the image is output. However, when a voltage is applied to the charging means, since a discharge start voltage is different depending on an ambient temperature or humidity, a thickness of a surface layer of the photosensitive member, or the like, the surface potential of the photosensitive member is different depending on conditions even if the same voltage is applied to the charging means. Moreover, since the sensitivity of the photosensitive member to the exposure means is different, the surface potential of the exposed photosensitive member is not constant even when the same exposure means is used. If the surface potential of the photosensitive member is not constant, the image density changes depending on conditions.
Thus, according to the invention disclosed in Japanese Patent Application Publication No. H5-66638, the surface potential of a photosensitive member is measured and fed back to control image formation so that a constant potential is always maintained. Moreover, according to the inventions disclosed in Japanese Patent Application Publication Nos. 2013-125097 and 2012-13381, a discharge start voltage is calculated, a surface potential is calculated from the information and is fed back to control image formation so that the surface potential of the photosensitive member is always constant.
The configuration proposed in Japanese Patent Application Publication No. H5-66638 can measure an accurate surface potential by directly measuring the surface potential. However, the space for installing a surface potentiometer is required, which may increase the apparatus size, and the installation cost for the surface potentiometer is required, which may increase the apparatus cost. The configuration proposed in Japanese Patent Application Publication Nos. 2013-125097 and 2012-13381 can measure the surface potential of a photosensitive member without adding a member to an image forming apparatus. However, the surface potential of a photosensitive member is preferably detected as quickly as possible.